When a modern gasoline engine and emissions system is retrofit with known gaseous alternative fuel systems, several problems occur during operation with alternative fuels such as methane or propane. Alternate fuel combustion chemistry is substantially different than gasoline combustion chemistry. The gasoline engine and its emissions system have been optimized for gasoline fuel combustion chemistry and optimized to provide the lowest possible emissions while providing maximum fuel economy. When a gasoline engine and emission system are operated on an alternative fuel, the emissions can be consequently higher than on gasoline and fuel economy can be poor. Cold weather starting and drivability are deficient in comparison with gasoline operation as a result of incorrect air-fuel mixtures.
The best available alternative fuels air-fuel mixing systems have wide variations in air-fuel ratio due to atmospheric pressure, repeatability, hysteresis, temperature, installation and numerous other factors. Available alternative fuel feedback control systems used in conjunction with alternative fuel mixers use a closed-loop limit-cycle controller (see FIGS. 1 and 2) to compensate for these variations. A helpful explanation of the closed-loop limit-cycle controller used in electronic emissions control systems is found in Ribbens, Wm. B. and N. P. Mansow, Understanding Automotive Electronics. Indianapolis, Howard W. Sams & Co., 1984, at pages 168-173.
A limit-cycle controller cycles about a set point of stoichiometry, but under normal driving conditions the optimum air-fuel ratio constantly changes as fuel demand and engine load vary. The limit-cycle controller is slow to respond to these changes with a resulting increase in overall exhaust emissions. Even at the steady state, operation about the set point of stoichiometry in accordance with the exhaust gas emission oxygen sensor in a modern gasoline system produces exhaust gas chemistry that is not compatible with the gasoline emission system catalytic converter and therefore produces excessively high emissions.
These high emissions can be reduced, with varying degrees of success, by biasing an air fuel ratio to shift the resulting emissions concentrations within an optimum operating region for a catalytic converter (see for example H. Shiga and S. Mizutani, Car Electronics, Warrendale, Pa., SAE, 1988, FIG. 3.9 at page 64, and Understanding Automotive Electronics, pages 163-166.)
U.S. Pat. No. 5,033,440 to Kumagai teaches use of a leanness bias in air-fuel ratio in an internal combustion engine when an airflow sensor detects low mass airflow.
U.S. Pat. No. 5,016,595 to Kawai et al. teaches use of a leanness bias at high values of mass airflow and also at high altitude.
A two-fold improvement in transient response, with corresponding reduction of unwanted emissions, is obtained by introducing the concept of a fuel wavefront and permitting the limit cycle to operate in a natural transport delay-oscillator mode (see FIG. 3).
U.S. Pat. No. 4,932,384 to Weingartner teaches use of periodic high frequency control intervals periodically inserted between a pair of transport delay control cycles during steady state conditions.
U.S. Pat. No. 4,926,826 to Nakaniwa teaches use of a conventional electronic air-fuel ratio control system. The control cycle is based on the transport delay oscillator. A first-order control term seeks to restore the air-fuel ratio to a desired target value. A second-order term, proportional to mass airflow and inversely proportional to engine speed, is used to improve transient response. Control is achieved by adjustment of the integration rate. The system employs a fuel wavefront.
It is an object of the present invention to improve the reduction of exhaust emissions on internal combustion engines operating on gaseous alternative fuels using a simple, low-cost air-fuel ratio controller.